1. Field of the Invention
The present invention relates to a phase-change optical recording medium on which information is recorded and reproduced by being irradiated with a light beam. In particular, the present invention relates to a phase-change optical recording medium which is capable of recording and reproducing information at a high speed.
2. Description of the Related Art
A phase-change optical recording medium is one of rewritable information-recording media. The phase-change optical recording medium comprises a recording layer in which the atomic arrangement is reversibly changed between two different states (between the amorphous and the crystal) by being irradiated with a light beam. Information is recorded on the basis of the two different states of the atomic arrangement of the recording layer. The phase-change optical recording medium is especially cheap among the rewritable media. Therefore, the phase-change optical recording medium has conspicuously come into widespread use for consumer products. In particular, the widespread use is being quickly developed for those concerning recording media for household or domestic video recording media. When the video recording media, which have been hitherto tapes, are replaced with disks of phase-change optical recording media, it is also possible to add new functions such as the time shift reproduction. Therefore, it is required to provide highly sophisticated characteristics exceeding the characteristics of the conventional phase-change optical recording medium having been required for the backup media for computers. For example, in the case of the time shift reproduction, it is necessary that the reproduction should be performed while following images having been just recorded. Therefore, the recording and the reproduction must be switched at a high speed every certain period of time. For this purpose, it is necessary to further increase the access speeds of the recording and the reproduction of information as compared with those having been hitherto used.
In the case of the conventional phase-change optical recording medium, information has been recorded and reproduced by controlling the number of revolutions of the medium in accordance with the CLV (Constant Linear Velocity) system. The CLV system is based on a control method in which the relative velocity (linear velocity) between the light beam and the medium is always constant. That is, in the case of the CLV system, the data transfer rate is always constant during the recording and reproduction. Therefore, it is possible to extremely simplify the signal processing circuit to be used for the recording and reproduction of information. However, in the case of the CLV system, it is necessary that the number of revolutions of the motor is adjusted depending on the radial position of the light beam on the medium so that the linear velocity is constant when the light beam is moved in the radial direction on the medium. Therefore, in the case of the CLV system, the access speed to record and reproduce information is consequently slow.
On the other hand, as for the CAV (Constant Angular Velocity) system which makes it possible to record and reproduce information while maintaining a constant number of revolutions of the medium, it is possible to perform the high speed access, because it is unnecessary to control the number of revolutions of the motor depending on the radial position. However, in the case of the CAV system, the data transfer rate differs depending on the radial position during the recording and reproduction. Therefore, the signal processing circuit, which is used to record and reproduce information, is complicated. Further, in the case of the CAV system, the linear velocity is increased toward the outer circumference of the disk. Therefore, it is necessary to quicken the crystallization speed of the recording layer on the outer circumferential side as compared with the inner circumferential side of the disk. Therefore, in the case of the CAV system, it is necessary to use any special recording layer which makes it possible for the crystallization speed of the recording layer to respond to both of the high linear velocity area on the outer circumferential side and the low linear velocity area on the inner circumferential side of the disk.
In the phase-change optical recording medium, a Ge—Sb—Te-based alloy is generally used as the phase-change material for the recording layer. In order to protect such a recording layer, protective layers, each of which is composed of a dielectric material, are formed on both sides of the recording layer in many cases. Further, in order to avoid the chemical reaction and the atomic diffusion at the interface between the recording layer and the protective layer, a phase-change optical recording medium has been also suggested, in which a barrier layer is provided between a recording layer and a protective layer (see, for example, WO97/34298, pp. 18–22, FIG. 2).
In the phase-change optical recording medium having the conventional recording layer based on the use of the Ge—Sb—Te-based phase-change material, the high speed of the crystallization speed of the recording layer is principally realized by adding Sn to the recording layer. However, in spite of the fact that the high speed of the crystallization speed can be realized by adding Sn to the recording layer, the melting point of the recording layer is consequently increased. Therefore, it is necessary to record information by using a light beam having a higher output. If information is repeatedly rewritten with the high output light beam, the following problem has been caused. That is, the information recording and reproducing characteristics (for example, the signal output, the jitter, the reflectance, and the recording sensitivity) are suddenly deteriorated as compared with a case in which a light beam having a low output is used. Specifically, the following problem arises. That is, if information is repeatedly rewritten with a high output light beam, then the difference in refractive index of the recording layer is decreased between the crystalline state and the non-crystalline state (amorphous state), and the output of the reproduced signal is consequently decreased.
The Ge—Sb—Te-based alloy, which is used for the phase-change recording material, has such a feature that the difference in refractive index between the crystal and the amorphous is increased as the composition of GeTe is increased. Therefore, in the case of the phase-change optical recording medium, the output of the reproduced signal is also increased as the composition of GeTe of the recording layer is increased. However, as shown in a phase diagram of GeTe—Sb2Te3 in FIG. 10 (see V. S. Zemskov et. al., Handbook of Semiconductor System Solid Solution, published by NISSO), the melting point of the Ge—Sb—Te-based alloy is raised as the composition of GeTe is increased in an area in which the composition of GeTe is not less than 50 mol. %. Therefore, if a phase-change material in this composition area is used as a recording layer, the repeated rewriting characteristic is deteriorated, because the melting point is raised as the composition of GeTe is increased. That is, in this case of the phase-change optical recording medium, when the composition of GeTe of the recording layer is increased, then the difference in refractive index between the crystal and the non-crystal is increased, and the reproduced signal output is increased. However, the melting point is raised, and the repeated rewriting characteristic is deteriorated. Therefore, the phase-change optical recording medium based on the use of the phase-change material in this composition area has involved such a problem that it is extremely difficult to satisfy both of the reproduced signal output characteristic and the repeated rewriting characteristic.
A principal cause of the deterioration of the repeated rewriting characteristic is as follows. That is, when information is repeatedly rewritten, the phase-change optical recording medium is repeatedly heated by the light beam. Therefore, the recording layer and the dielectric protective layer composed of ZnS—SiO2 or the like adjoining the recording layer undergo the following interaction. That is, the constitutive elements of one of the both layers mutually make invasion or diffusion into the other, and/or the constitutive elements of the both layers mutually cause any chemical reaction. In order to avoid the invasion, the diffusion, and the chemical reaction of the constitutive elements of the both layers, for example, a method is suggested in WO97/34298 (pp. 18–22, FIG. 2), in which a nitride of Ge—N, Ge—Cr—N or the like is interposed as an interface layer between the recording layer and the dielectric protective layer. Phase-change optical recording media, each of which is provided with the interface layer as described above, are disclosed, for example, in Japanese Patent Application Laid-open Nos. 10-289478, 11-167746, 11-238249, 11-339316, 2001-126312, 2002-74739, and 2002-74747.
At present, it is requested for the phase-change optical recording medium to further realize a high density of information and a high speed of recording and reproduction. In order to respond to this request, it is necessary to use a phase-change material which has a melting point of a recording layer higher than those hitherto used. For example, Ge2Sb2.3Te5 (relative ratio), which is used as a phase-change material in WO97/34298 (pp. 18–22, FIG. 2), has a melting point of about 630° C. However, for example, Bi7Ge41Te52 (at. %), which is suitable for the realization of high speed, has a melting point of at least not less than 700° C. As shown in a phase diagram of GeTe—Bi2Te3 in FIG. 11 (see V. S. Zemskov et. al., Handbook of Semiconductor System Solid Solution, published by NISSO), the following fact has been revealed in the same manner as in the Ge—Sb—Te system. That is, the melting point is raised as the ratio of GeTe is increased in an area in which the ratio of GeTe is not less than about 25 mol % in the Bi—Ge—Te-based alloy as well. It is doubtless that the change will be made in such a tendency that the ratio of GeTe is further increased, i.e., the composition will be changed toward the Ge-rich area in future in the case of the phase-change material of the Ge—Sb—Te system and the Bi—Ge—Te system. It is inevitable that the melting point of the recording layer will be further raised in the phase-change optical recording medium. As for the phase-change optical recording medium based on the use of the high melting point phase-change material as described above, it is considered that any excellent repeated rewriting characteristic is not obtained with the interface layer of Ge—N or Ge—Cr—N having been hitherto used for the low melting point phase-change optical recording medium.